1. Field of the Invention
The present invention relates generally to an active piezoelectric energy harvester with an embedded variable capacitance layer and a method of manufacturing the same and, more particularly, to an active piezoelectric energy harvester with an embedded variable capacitance layer and a method of manufacturing the same, in which a direct current voltage applied to the embedded variable capacitance layer is controlled to precisely adjust a resonance frequency in real time.
2. Discussion of Related Art
Piezoelectric energy harvesters are elements that cause bending, contraction, extension, etc. in a piezoelectric layer using vibration or pressure and thus generate an alternating current voltage by a piezoelectric effect.
These piezoelectric energy harvesters are variously applied in that they can use pressure or vibration caused by exercise of a person, pressure or vibration caused by a vehicle such as a car, and pressure or vibration caused by a natural environment.
FIG. 1 illustrates a conventional cantilever type piezoelectric energy harvester.
Referring to FIG. 1, a conventional cantilever type piezoelectric energy harvester 100 is configured such that, when one free end thereof is displaced by external vibration, a stress is applied to a piezoelectric layer 110 above a fixing layer 130, and thereby changes the position of an internal dipole, and thus an alternating current voltage is induced to piezoelectric layer electrodes 120a and 120b. 
Here, the alternating current voltage induced to the piezoelectric layer electrodes 120a and 120b has a polarity determined by whether the stress applied to the piezoelectric layer 110 is a tensile or compressive stress.
This induced alternating current voltage is converted into a direct current voltage by a rectifier circuit 140, undergoes adjustment in its intensity through a control circuit 150, and is stored in a storage device, such as battery and capacitor 160.
However, since the magnitude of the external vibration applied to the piezoelectric energy harvester may vary frequently, a means capable of controlling the resonance frequency of the piezoelectric energy harvester according to the frequency of the external vibration is required.
To this end, a method of varying the mass of a balance weight 170, which is attached to the free end, the cantilever tip, of the piezoelectric energy harvester, or the length of the cantilever itself to control the resonance frequency is used.
However, this method has a problem in that it is difficult to precisely control the resonance frequency, as well as that it is impossible to control the resonance frequency in real time during energy conversion.
To solve this problem, a method of connecting a variable capacitor to the outside of the piezoelectric energy harvester to control the resonance frequency has been suggested.
In the case of this method, since the variable capacitor is separately provided, the size of the entire element is increased.